The qualitative and quantitative determination of cations is of major significance in areas such as chemical and biochemical engineering for process control, in agriculture chemistry for soil research and fertilizer metering and in medicine for diagnostic and therapeutic determination of the potassium-sodium ratio. Present methods for cation determination include flame photometry and atomic absorption spectroscopy, both of which require sophisticated apparatus. Ion-sensitive cation electrodes on an ion-exchange basis generally yield sufficiently differentiated results, but are cumbersome to use.
Vogtle, U.S. Pat. No. 4,367,072 describes a process for determining ions. It is essentially based on ion-selective complexing between the ion to be determined and a complexing agent and measurement of the extinction change occurring during complexing. The complexing agent is bonded with a chromophore.
The selective complexing agent may be an oligoether, oligoester or oligoamide, containing, for example, corand, cryptand or cyclic peptide and/or polyethylene glycol groups or other hetero atom-containing groups. The covalently or heteropolarly bound chromophore is a dye or fluorescent dye or a chromogen whose absorption spectra change through interaction with non-polar ions or lipophilic molecules through charge shifts or disturbances of the mesomeric system. This principle is well known in nature and in the art. Hemin, chlorophyll and metal complex dyes and metal indicators (e.g., xylenol orange and methylthymol blue based on the colorless complexing agent ethylenediaminetetraacetic acid (EDTA)) exhibit, to a greater or lesser extent, this general configuration.
A general problem of the above-cited complexing agents is that they usually are capable of reacting only in organic media, whereas the ion being determined is, as a rule, present in an aqueous solution. Although the aqueous solutions of the ions could be transformed in many cases to organic media by concentration, inclusion in organic salts, or solvent extraction, this would not satisfy the requirements of a practical and, if necessary, automated rapid method.
Klink, et al., European Patent Publication 85,320, disclose a potassium reagent and a procedure for determining potassium ions. The reagent contains a compound of general formula ##STR4## where n and m=0 or 1, X=N or COH and R=p-nitrophenylazo, 3-phenylisothiazolyl-5-azo, isothiazolyl-5-azo, thiazolyl-5-azo, 2,4,6-trinitrophenylazo, p-nitrostyryl, p-benzoquinonemonoimino and bis- (p- dimethylaminophenyl) hydroxy-methyl. The potassium ions are determined in a reaction medium consisting of water and at least one water-miscible organic solvent and in the presence of an organic base.
Klink et al. do not recognize the need for sodium ion masks in EP 85,320. They provide extinction maxima data of various cations, and state that aside from the extinction maxima for rubidium, all other extinction maxima for the various cations tested are so far removed from potassium's extinction maxima that no interference occurs. However, Klink et al. base their conclusion on data obtained from isolated cation measurements, and fail to contemplate the effect on extinction maxima for these cations in mixed cation solutions. Contrary to the conclusion reached by Klink et al., a sodium ion mask greatly enhances determination of potassium ions using chromogenic cryptands described in EP 85,320.
The present invention is directed to compounds, reagents and methods which permit rapid determination of cations in single-phase aqueous media, wherein the improvement comprises use of one or more interfering cation complexing compound masks.